Field of the Invention
The present invention relates to a zoom lens incorporated in pickup apparatuses such as a digital still camera, a TV camera, and a video camera.
Description of the Related Art
Related Background Art
The zoom lens as mentioned above has a four unit structure in which a first lens unit having a positive refractive power for focus adjustment, a second lens unit having a negative refractive power for power-varying, a third lens unit having a positive or negative refractive power for compensating image-plane fluctuation in accordance with the power-varying, and a fourth lens unit having a positive refractive power for forming an image are arranged from the order of an object, or a three unit structure in which the third lens unit has a function of compensating image-plane fluctuation and forming an image.
Of such zoom lenses, in particular, a telescopic-type zoom lenses, each having an angle of view (2ω) not greater than 2.0 degrees at its telephoto end and a variable power ratio of about 1.5 to 3, are disclosed in U.S. Pat. No. 5,442,486, Japanese Patent Laid-Open No. 10-90599, and European Patent Application No. 1092999. Also, a super-telescopic-type zoom lens for use in a TV camera, having an angle of view (2ω) not greater than 2.0 degrees at the telephoto end and a variable power ratio of about 40 is disclosed.
When an object such as an animal or a bird is picked up outdoors at a long range so as to be provided for a TV nature program or the like, a zoom lens having a high variable power ratio (for example, having a large magnification not smaller than 5 and an angle of view not greater than 2 degrees (that is, having a focal length not shorter than 330 mm when converted to the corresponding one of a ⅔ type CCD) and a high optical performance is desired. Also, in such a pickup activity, since a camera is often used while being shouldered, a more compact and lighter-weight zoom lens easy to be taken anywhere is desired.
With respect to a zoom lens having the four unit structure, the following expressions are satisfied:
Expressions 1fw=f1×β2w×β3w×β4  (a-1)fT=f1×β2T×β3T×β4  (a-2)Z=fT/fw=β2T/β2w×β3T/β3w  (a-3)Wherein, fw and fT respectively represent the focal lengths of the overall system of the zoom lens at its wide angle and telephoto ends, f1 represents the focal length of the first lens unit, Z represents the variable ratio of the zoom lens, βiw and βiT respectively represent the image-forming magnifications of an i-th lens unit at its wide angle and telephoto ends, and β4 represents the image-forming magnification of the fourth lens unit, which is constant during power-varying since the fourth lens unit is fixed during the power-varying.
In order to achieve a telescopic feature, it is sufficient to make larger the focal length f1 of the first lens unit or the image-forming magnifications β2T, β3T, and β4 of the second, third, and fourth lens units at the telephoto end according to the expression (a-2).
However, when β2T, β3T, and β4 are made greater, spherical and longitudinal chromatic aberrations generated in the first lens unit increase in proportion to the square of each image-forming magnification. Hence, it is better to make the focal length f1 of the first lens unit larger, especially from the viewpoint of an optical performance at the telephoto end. Meanwhile, the expressions (a-1) to (a-3) can also be applied to a zoom lens having the three unit structure except for β4.
FIG. 18 is a conceptual view of a zoom lens having the four unit structure, illustrating a first lens unit L1 fixed during power-varying and having a positive refractive power and a second lens unit L2 moving toward an image plane upon the power-varying. In the figure, a reference character N represents the image point of the first lens unit L1, that is, the apparent object point of the second lens unit L2; f1 and f2 respectively represent the focal lengths of the first and second lens units L1 and L2; reference characters e1w and e1T respectively represent principal-point intervals between the first and second lens units L1 and L2 at the wide angle and telephoto ends of the zoom lens; and a reference character mv represents a moving amount of the second lens unit L2 from the wide angle to telephoto ends of the zoom lens.
In order to achieve a larger magnification, it is needed to make the ratio of the image-forming magnifications of the second and third lens units at the wide angle and telephoto ends of the zoom lens greater according to the expression (a-3). In particular, when attention is focused on a change of the image-forming magnifications β2 of the second lens unit L2 largely affecting the variable ratio, the following expressions are brought about:
                    Expressions        ⁢                                  ⁢        2                                                                      β          ⁢                                          ⁢          2          ⁢          w                =                  f2                      f1            -            e1w            +            f2                                              (                  b          ⁢                      -                    ⁢          1                )                                          β2          ⁢                                          ⁢          T                =                              f2                          f1              -              e1T              +              f2                                =                      f2                          f1              -              e1w              -              mv              +              f2                                                          (                  b          ⁢                      -                    ⁢          2                )                                                      β2            ⁢                                                  ⁢            T                                β2            ⁢                                                  ⁢            w                          =                              f1            -            e1w            +            f2                                f1            -            e1w            -            mv            +            f2                                              (                  b          ⁢                      -                    ⁢          3                )            
According to the expression (b-3), the conditions for achieving a larger magnification are given as below,    (1) The focal lengths f1 and f2 of the first and second lens units L1 and L2 are small;    (2) the moving amount mv of the second lens L2 is large;    (3) the principal-point interval e1w between the first and second lens units L1 and L2 at the wide angle end is large.
Unfortunately, making smaller the focal length f1 of the first lens unit L1 requires the image-forming magnifications of the second and following lens units to be larger, thereby increasing spherical and longitudinal chromatic aberrations and deteriorating the optical performance. Also, making smaller the focal length f2 of second lens unit L2 increases fluctuation of various aberrations in accordance with the power-varying, thereby deteriorating optical performance.
In addition, making larger the moving amount mv of the second lens unit L2 requires a wider space for the second lens unit L2 to move, thereby resulting in increase in the overall length of the zoom lens. Further, although making larger the principal-point interval e1w between the first and second lens units L1 and L2 is advantageous for a greater magnification, a large space is generated at the wide angle end between the first and second lens units L1 and L2, thereby resulting in increases in the overall length of the zoom lens and the diameter of a front cell lens unit of the zoom lens.
Accordingly, in order to achieve a compact and light-weight zoom lens having a large magnification and a high performance, it is important to set parameters including the foregoing f1, f2, mv, and e1w in appropriate ranges in accordance with the specifications of the lens. Also, since the principal-point interval e1w between the first and second lens units L1 and L2 largely affects achievement of a greater magnification, the overall length of the lens, and the diameter of the front cell lens unit, it is especially essential to appropriately set the principal point of the first lens unit L1.
In the zoom lens disclosed in U.S. Pat. No. 5,442,486, the focal length f1 of the first lens unit L1 is about 600 mm to 900 mm, the principal-point interval e1w between the first and second lens units L1 and L2 is about 450 to 700 mm, and the moving amount mv of the second lens unit L2 is about 10 to 15 mm. Also, in the zoom lens disclosed in Japanese Patent Laid-Open No. 10-90599, the focal length f1 of the first lens unit L1 is about 235 mm, the principal-point interval e1w between the first and second lens units L1 and L2 is about 110 mm, and the moving amount mv of the second lens unit L2 is about 45. Thus, in both zoom lenses, since mv is smaller than (f1−e1w) according to the expression (b-3), the variable ratio of the zoom lens is as small as about 1.5 to 3, thereby failing to satisfy the requirement of a greater magnification.
In the zoom lens disclosed in EP1092999, although the variable ratio of the zoom lens is not smaller than 40, the moving amount mv of the second lens unit L2 is as large as about 100 to 120 mm, thereby resulting in a larger-sized zoom lens having the overall length of about 400 mm and the diameter of about 180 mm of the front cell lens unit.